 Improvisation is composing on the spot, when a singer or instrumentalist invents melodies and lines over a chord progression played by rhythm section instruments (piano, guitar, double bass) and accompanied by drums.Although blues, rock, and other genres use improvisation, it is done over relatively simple chord progressions which often remain in one key (or closely related keys using the circle of fifths, such as a song in C Major modulating to G Major).Jazz improvisation is distinguished from this approach by chordal complexity, often with one or more chord changes per bar, altered chords, extended chords, tritone substitution, unusual chords (e.g., augmented chords), and extensive use of ii–V–I progression, all of which typically move through multiple keys within a single song.